Laser Welding Process
There are literally thousands of references on the theory and practical uses of lasers. They are used in everything from portable CD players to sophisticated weapons systems. The term LASER is an acronym for "Light Amplification by Stimulated Emission of Radiation," and is defined as "any of several devices that emit highly amplified and coherent radiation of one or more discrete frequencies." At Northeast Laser & Electropolish, we utilize pulsed Nd:Yag (Neodymium-Doped Yttrium-Aluminum-Garnet) type lasers for welding. The Nd:Yag rod, when stimulated by a flash lamp, emits light in the infrared range with a wavelength of 1.06 microns. This light is then focused and delivered to the workpiece, where the high energy density beam is used to weld.
Delivery of Laser Energy
There are two ways that the laser beam is delivered to the workpiece. The first involves the use of "hard optics," and the second involves the use of a fiber optic cable. "Hard optics" basically means that the laser beam is deflected and focused through the use of mirrors and lenses only. This method has practical limitations in the distance of the workpiece from the laser source and dictates that it be moved into the correct position and angle to perform the weld. This type of workstation is ideal for many small or delicate items that require manual or single "spot" welds. The second delivery method involves the use of a fiber optic cable. The laser energy can be focused into one end of the cable and emerge at the other end (tens of meters away), with a minor loss of energy. The beam can then be "collimated" and refocused onto the workpiece. This method allows for the beam to be delivered precisely to the needed area, and even allows for movement of the focusing optics instead of, or in addition to the workpiece itself. At Northeast Laser, we have both "hard optics" and fiber delivered laser systems to suit just about any application.
Welding with Laser Energy
Up to the point that the laser beam contacts the workpiece, all the components that direct it are either transparent, refractive or reflective, absorbing only small amounts of energy from the ultraviolet light. The laser power supply is capable of delivering a "pulse" of light that has accurate and repeatable energy and duration. When the "pulse" of laser energy is focused into a small spot (adjustable anywhere from approximately 0.1 to 2.0 mm in diameter) onto the workpiece, the energy density (energy/area) becomes quite large. The light is absorbed by the (metal) workpiece, causing a "keyhole" effect as the focused beam "drills" into, vaporizes and melts some of the metal. As the pulse ends, the liquefied metal around the "keyhole" flows back in, solidifying and creating a small "spot" weld. The entire process takes only milliseconds. The laser has the ability to fire many pulses per second, and moving the workpiece or optics allows anything from separate "spot" welds to a series of overlapping "spot" welds to create a "seam" weld that can be structural and/or hermetic.
Similarities and Differences to Other Welding Processes
When compared to other welding processes, laser welding has some similar as well as some unique characteristics Like GTAW (Gas Tungsten Arc Welding), laser welding is a fusion process performed under inert cover gas, where filler material is most times not added. Like electron beam welding, Laser welding is a high energy density beam process, where energy is targeted directly on the workpiece. Laser differs from both GTAW and EB (electron beam) welding in that it does not require that the workpiece complete an electrical circuit. And since electron beam welding must be performed inside a vacuum chamber, laser welding can almost always offer a cost advantage over EB in both tooling and production pricing.
Advantages of Laser Welding
One of the largest advantages that pulsed laser welding offers is the minimal amount of heat that is added during processing. The repeated "pulsing" of the beam allows for cooling between each "spot" weld, resulting in a very small "heat affected zone". This makes laser welding ideal for thin sections or products that require welding near electronics or glass-to-metal seals. Low heat input, combined with an optical (not electrical) process, also means greater flexibility in tooling design and materials.
Joint Types and Tooling Concerns
Whether through part design, tooling design, or a combination of both, one of the most important factors for a successful laser weld is that components be held in intimate contact along the weld area. The ideal weld joint should have no gap between components. This is especially true in a lap weld joint configuration. Even the slightest space between parts can be the difference between a consistently strong weld, and no weld at all. Butt or seam weld joints are slightly more tolerant, where successful welding can be performed with up to 0.025mm (0.001 inch) separation, and in some cases (depending on section thickness and joint design) with gaps as large as 0.05mm (0.002 inch). Fillet welds can also offer challenges, especially when welding two parts at a 90-degree angle. Since laser welding is most often done without the benefit of filler metal, the material that forms the fillet must be "drawn" from the two sections being welded. This can often cause stress cracking that starts at the toe of the weld and propagates through the joint, causing weakness or creating a "leak path" through joints that need to be hermetic. There are several weld joint design features that should be avoided/exploited in order to ensure a consistent weld in production situations. The engineers at Northeast Laser are available to discuss these various features and suggest which ones may be suited to your application.
Although laser welding is applicable to a large range of both ferrous and non-ferrous metals, there are some materials and combinations of materials that perform better than others. For instance, 304 and 304L series stainless weld extremely well, while 303, 316, and 316L stainless are crack-sensitive. Since 303 stainless is often used because of its machineability, it is sometimes possible to make one component from 303, and the (less complex) mating component from 304L. The resulting alloy is usually less sensitive to cracking during welding. The laser process can also be applied to titanium, kovar, copper and certain aluminums, though copper and aluminum require much more energy due to their reflective and heat transfer characteristics. Laser welding can be used to join dissimilar metals as well, such as copper to stainless, or stainless to certain types of phosphor bronze. There are many combinations that work well, while others should be avoided.
Finally, no discussion on welding would be complete without mentioning concerns about defects or unsuccessful weld results. Cracking, burning, incorrect weld depth and welds off the joint line are some of the most common and detrimental issues. There are many factors that can cause issues in a metal joining process, only some of which are under the control of the process itself. Once successful laser welding parameters are developed for a particular application, the issue then becomes a matter of process control. Most up to date laser systems, like those employed by Northeast Laser, automatically control the power and duration of each laser pulse. Cleanliness of parts, cover gas, tooling and motion control are some of the other parameters that must be monitored and regulated throughout the production run. Out of tolerance parts, inferior or contaminated materials, incorrect joint design, and parts with defects are not within the control of the weld process, but can result in inconsistent or unreliable welds that can be misdiagnosed as process problems. At Northeast Laser, we know that working with the customer in order to minimize time and expense, while maximizing quality and service is the surest way to a successful vendor/customer relationship.